U.S. patent application number 10/990117 was filed with the patent office on 2005-06-16 for acoustic vibration generating element.
This patent application is currently assigned to NEC TOKIN CORPORATION. Invention is credited to Tamura, Mitsuo.
Application Number | 20050129257 10/990117 |
Document ID | / |
Family ID | 34510550 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050129257 |
Kind Code |
A1 |
Tamura, Mitsuo |
June 16, 2005 |
Acoustic vibration generating element
Abstract
In an acoustic vibration generating element, a piezoelectric
bimorph element or unimorph element is covered with a covering
member of a flexible material at least on two surfaces
perpendicular to a thickness direction. The covering member may be
provided with a plurality of V-shaped grooves so as to improve a
generated vibrating force. Alternatively, the covering member may
be provided with an air chamber in the vicinity of a surface of one
side so as to prevent sound leakage. Further, the covering member
and an earhook may be integrally formed by the flexible material so
as to achieve a light-weight acoustic vibration generating element
suitable for a bone conduction speaker.
Inventors: |
Tamura, Mitsuo; (Sendai-shi,
JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
NEC TOKIN CORPORATION
Sendai-shi
JP
|
Family ID: |
34510550 |
Appl. No.: |
10/990117 |
Filed: |
November 15, 2004 |
Current U.S.
Class: |
381/151 ;
600/312; 600/330 |
Current CPC
Class: |
H04R 2499/11 20130101;
H04R 17/10 20130101; H04R 2460/13 20130101; H04R 17/00
20130101 |
Class at
Publication: |
381/151 ;
600/312; 600/330 |
International
Class: |
H04R 025/00; A61B
005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2003 |
JP |
414064/2003 |
Claims
What is claimed is:
1. A bone conduction acoustic vibration generating element
comprising a piezoelectric bimorph or unimorph element and covering
members made of a flexible material and attached to at least two
surfaces perpendicular to a thickness direction of the
piezoelectric bimorph or unimorph element.
2. The bone conduction acoustic vibration generating element
according to claim 1, wherein each of the covering members is
provided with a plurality of grooves formed on its surface.
3. The bone conduction acoustic vibration generating element
according to claim 1, wherein one of the covering members is
provided with an air chamber.
4. The bone conduction acoustic vibration generating element
according to any one of claims 1 to 3, wherein the piezoelectric
bimorph element has a laminated structure comprising piezoelectric
ceramics sheets and internal electrodes.
5. The bone conduction acoustic vibration generating element
according to any one of claims 1 to 3, further comprising an
earhook portion made of the flexible material and integrally formed
with the covering member.
6. The bone conduction acoustic vibration generating element
according to claim 4, further comprising an earhook portion made of
the flexible material and integrally formed with the covering
member.
7. A bone conduction acoustic vibration generating element
comprising a piezoelectric bimorph or unimorph element and a
covering member made of a flexible material and covering a whole of
the piezoelectric bimorph element or the piezoelectric unimorph
element.
8. The bone conduction acoustic vibration generating element
according to claim 7, wherein the covering member is provided with
a plurality of grooves formed on its surface.
9. The bone conduction acoustic vibration generating element
according to claim 7, wherein the covering members is provided with
an air chamber formed on one side thereof.
10. The bone conduction acoustic vibration generating element
according to any one of claims 7 to 9, wherein the piezoelectric
bimorph element has a laminated structure comprising piezoelectric
ceramics sheets and internal electrodes.
11. The bone conduction acoustic vibration generating element
according to any one of claims 7 to 9, further comprising an
earhook portion made of the flexible material and integrally formed
with the covering member.
12. The bone conduction acoustic vibration generating element
according to claim 10, further comprising an earhook portion made
of the flexible material and integrally formed with the covering
member.
Description
[0001] This invention claims priority to prior Japanese patent
application JP 2003-414064, the disclosure of which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an acoustic vibration generating
element. In particular, this invention is suitable for a bone
conduction device, such as a bone conduction speaker, for
converting an acoustic electric signal into acoustic vibration to
be transmitted to a part of a human body, such as a cranial bone or
an arm, so that an acoustic sound is sensed by an auditory
nerve.
[0003] Heretofore, as an electromechanical transducer for a bone
conduction device, use is predominantly made of an electromagnetic
type. The electromechanical transducer of the electromagnetic type
utilizes a principle same as that of a dynamic speaker and converts
a driving force generated by interaction between an electric
current flowing through a coil and a magnet into mechanical
vibration. The electromechanical transducer of the type is
disclosed, for example, in Japanese Patents (JP-B) Nos. 2967777
(corresp. to U.S. Pat. No. 6,141,427) and 3358086 (corresp. to U.S.
Pat. No. 6,668,065).
[0004] However, the electromechanical transducer of the
electromagnetic type is disadvantageous in the following respects.
The electromechanical transducer of the electromagnetic type
generates an electromagnetic force and therefore requires the
electric current. When the electric current flows through the coil,
an energy loss inevitably occurs by a resistance of the coil. Thus,
most of energy supplied from a power source is dissipated at the
coil as Joule heat and only about 1% of the energy is used as
acoustic energy. Further, in a low-frequency region, the electric
current tends to be excessive because of low impedance so that a
load upon the power source is increased. As a result, a sound
output level is inevitably limited in the low-frequency region.
Thus, in the low-frequency region, the sound output level tends to
be insufficient.
[0005] On the other hand, proposal is made of a transducer for a
bone conduction device, i.e., a bone conduction transducer which
uses a piezoelectric element, although in a minority. The bone
conduction transducer using the piezoelectric element comprises, as
an acoustic vibration generating element, a piezoelectric unimorph
element which is often used as a piezoelectric sound generator. The
piezoelectric unimorph element comprises a metal plate and a
piezoelectric plate adhered thereto. The bone conduction transducer
of the type is disclosed, for example, in Japanese Patent
Application Publications (JP-A) Nos. S59-140796 and S59-178895.
[0006] However, the bone conduction transducer using the
piezoelectric element is disadvantageous in the following respects.
Specifically, the bone conduction transducer using the
piezoelectric element has a resonance frequency of 1 kHz or more if
the bone conduction transducer has a practical size. Therefore,
reproduction in the low-frequency region lower than the resonance
frequency is insufficient. Further, since a mechanical quality
factor Q of a vibrating system is high, generation of vibration is
emphasized or attenuated at a specific frequency. In this event,
sound reproduction can not naturally and normally be carried
out.
[0007] As an example of the bone conduction device, there is also
provided a bone conduction speaker not for a hearing-impaired
person but for an unimpaired person. The bone conduction speaker of
the type is required to prevent a reproduced sound from leaking to
others except a user. However, with a known structure of the bone
conduction speaker, vibration of a vibration source propagates to a
structural member. As a result, the vibration of the structural
member is propagated to the surroundings as the reproduced
sound.
[0008] In bone conduction applications of the piezoelectric
element, a resonance frequency of the piezoelectric element must be
as low as possible if the low-frequency region is regarded as
important. In order to lower the resonance frequency of the
piezoelectric element, the following techniques A to C are
proposed.
[0009] A. To increase a diameter or a length of the piezoelectric
element, which determines a vibration mode.
[0010] B. To lower a flexural modulus K of the piezoelectric
element.
[0011] C. To add a mass to an antinode of vibration.
[0012] However, if an object equipped with the piezoelectric
element is a portable apparatus such as a mobile phone and,
therefore, the size of the piezoelectric element is restricted, the
technique A has limitations.
[0013] The technique B is achieved by reducing the thickness of a
piezoelectric ceramics sheet in case of a piezoelectric unimorph
element and by reducing the thickness of a metal plate (shim plate)
interposed between two piezoelectric ceramics sheets in case of a
piezoelectric bimorph element. However, in this technique, the
mechanical strength of the piezoelectric element is lowered. In
addition, the weight of the piezoelectric element itself is
decreased so that the resonance frequency is increased. Therefore,
no substantive effect is obtained. Alternatively, by selecting an
organic material having a small elastic modulus as the shim plate,
the flexural modulus K can be lowered to some extent. However, the
organic material generally has a small specific gravity so that the
weight of a whole of the piezoelectric element is decreased.
Therefore, the resonance frequency tends to be increased.
[0014] The technique C of adding the mass is disadvantageous in
that the mechanical strength tends to be weakened against shocking
vibration.
[0015] In a piezoelectric transducer such as the above-mentioned
bone conduction transducer using the piezoelectric element,
mechanical vibration is driven by piezoelectric distortion which is
caused by an electric voltage. Therefore, the piezoelectric
transducer is not accompanied with dissipation of Joule heat by the
coil in the above-mentioned electromechanical transducer of the
electromagnetic type. Therefore, it is possible to achieve energy
saving. In addition, since metal components such as a magnet and a
yoke are not required, a light weight and a thin profile can be
achieved. Thus, the piezoelectric transducer has many advantages.
In order to fully enjoy those advantages, the piezoelectric
transducer is required to overcome the disadvantages such as a high
resonance frequency and a high mechanical quality factor Q.
[0016] On the other hand, prevention of sound leakage to the
surroundings is an unavoidable issue in order to bring the bone
conduction speaker into practical use, whether electromagnetic or
piezoelectric. In order to further exhibit the characteristics of
the piezoelectric transducer, an input driving voltage is
preferably suppressed as low as possible. In this event, energy
loss of a driving circuit combined with the piezoelectric
transducer is advantageously suppressed.
[0017] Further, the bone conduction speaker is generally attached
to a human head when it is used. Therefore, it is desired for a
user that the bone conduction speaker is light in weight and is
easily wearable.
SUMMARY OF THE INVENTION
[0018] It is therefore an object of this invention to achieve a
lower resonance frequency, a lower mechanical quality factor Q, and
suppression of sound leakage in a bone conduction device, in
particular, a bone conduction speaker.
[0019] A bone conduction acoustic vibration generating element
according to this invention comprises a piezoelectric bimorph or
unimorph element and a covering or coating member made of a
flexible material and covering at least two surfaces perpendicular
to a thickness direction of the piezoelectric bimorph or unimorph
element.
[0020] In the bone conduction acoustic vibration generating element
according to this invention, a whole of the piezoelectric bimorph
element or the piezoelectric unimorph element may be covered with
the covering member.
[0021] In the bone conduction acoustic vibration generating element
according to this invention, the piezoelectric bimorph element may
have a laminated structure comprising piezoelectric ceramics sheets
and internal electrodes.
[0022] In the bone conduction acoustic vibration generating element
according to this invention, the covering member may be provided
with a plurality of grooves formed on its surface.
[0023] In the bone conduction acoustic vibration generating element
according to this invention, the covering member may be provided
with an air chamber formed on one side thereof.
[0024] The bone conduction acoustic vibration generating element
according to this invention may have an earhook portion made of the
flexible material and integrally formed with the covering
member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A is a perspective view of an acoustic vibration
generating element according to this invention in case where a
piezoelectric element has opposite surfaces covered with a flexible
material;
[0026] FIG. 1B is a perspective view of another acoustic vibration
generating element according to this invention in case where a
piezoelectric element is entirely covered with a flexible
material;
[0027] FIG. 2 is a sectional view of the acoustic vibration
generating element in FIG. 1A in case where the piezoelectric
element is a piezoelectric bimorph element;
[0028] FIG. 3 is a characteristic chart showing the change in
resonance frequency of the acoustic vibration generating element in
FIG. 2 when a silicone rubber as a covering member is changed in
thickness;
[0029] FIG. 4 is a perspective view of a piezoelectric bimorph
element having a laminated structure which may be used in this
invention;
[0030] FIG. 5 is a characteristic chart showing a result of
comparison of acceleration in an artificial internal ear between an
acoustic vibration generating element covered with a flexible
material according to a first embodiment of this invention and an
acoustic vibration generating element without being covered with
the flexible material;
[0031] FIG. 6 is a characteristic chart showing a result of
comparison of acceleration in an artificial internal ear between an
acoustic vibration generating element covered with a flexible
material according to a second embodiment of this invention and an
acoustic vibration generating element without being covered with
the flexible material;
[0032] FIG. 7 is a perspective view of an acoustic vibration
generating element according to a third embodiment of this
invention in which V-shaped grooves are formed on a surface of a
flexible material;
[0033] FIG. 8 is a view showing a result of comparison of
acceleration in an artificial internal ear between the acoustic
vibration generating element in FIG. 7 and an acoustic vibration
generating element without the V-shaped grooves;
[0034] FIG. 9 is a sectional view of an acoustic vibration
generating element according to a fourth embodiment of this
invention in which an air chamber is formed on one side of a
flexible material;
[0035] FIG. 10 is a view showing an acoustic vibration generating
element according to a fifth embodiment of this invention in which
a covering member of a flexible material and an earhook are
integrally formed; and
[0036] FIG. 11 is a view showing the acoustic vibration generating
element in FIG. 10 when it is attached to a human ear.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] Now, description will be made of this invention with
reference to the drawing.
[0038] At first referring to FIGS. 1A, 1B, 2, and 3, a basic
structure and a principle of this invention will be described in
detail.
[0039] Referring to FIG. 1A, an acoustic vibration generating
element according to this invention comprises a piezoelectric
bimorph element (or piezoelectric unimorph element) 1-1 and a pair
of covering members 1-2 attached to two surfaces perpendicular to a
thickness direction of the piezoelectric bimorph element 1-1.
[0040] Referring to FIG. 1B, another acoustic vibration generating
element according to this invention comprises the piezoelectric
bimorph element (or piezoelectric unimorph element) 1-1 entirely
covered with a covering member 1-2'. The covering members 1-2 and
1-2' are made of a flexible material.
[0041] Referring to FIG. 2, the acoustic vibration generating
element in FIG. 1A comprises the piezoelectric bimorph element 1-1
and the covering members 1-2 attached to opposite surfaces thereof,
i.e., two surfaces perpendicular to the thickness direction of the
piezoelectric bimorph element 1-1. The piezoelectric bimorph
element 1-1 comprises two piezoelectric ceramics sheets 1-11 and a
shim plate 1-12 interposed therebetween.
[0042] Referring to FIG. 2, the principle of this invention will be
described. For convenience of description, it is assumed that the
piezoelectric bimorph element 1-1 has a rectangular shape. The
piezoelectric bimorph element 1-1 has a resonance frequency F.sub.r
which is different depending upon a supporting structure. If
opposite ends of the piezoelectric bimorph element 1-1 are free
ends, the resonance frequency F.sub.r is given by the following
Equation (1). In Equation (1), .alpha. is a value determined by a
vibration mode and is equal to 4.73 in primary resonance. L
represents a length of the piezoelectric bimorph element 1-1, K, a
flexural modulus, .rho.S, a weight per unit length. 1 F r = 2 2 L 2
K S ( 1 )
[0043] The flexural modulus K of the acoustic vibration generating
element is determined by various factors such as the size and the
material of each of the piezoelectric ceramics sheets 1-11 and the
shim plate 1-12 forming the piezoelectric bimorph element 1-1, and
the covering members 1-2. Specifically, the flexural modulus K is
determined by the width w of each of the piezoelectric ceramics
sheets 1-11 and the shim plate 1-12, the thickness t.sub.c and the
elastic modulus E.sub.C of each of the piezoelectric ceramics
sheets 1-11, the thickness 2t.sub.s and the elastic modulus E.sub.s
of the shim plate 1-12, and the thickness t.sub.p and the elastic
modulus E.sub.p of each of the covering members 1-2 and is given by
the following Equation (2). 2 K = 2 w 3 { E s t s 3 + E c ( 3 t s 2
t c + 3 t s t c 2 + t c 3 ) + E p ( 3 t s 2 t p + 6 t s t c t p + 3
t c 2 t p + 3 t s t p 2 + 3 t c t p 2 + t p 3 } ( 2 )
[0044] The weight .rho.S per unit length is determined by the
thickness t.sub.c, 2t.sub.s, and t.sub.p and the specific gravity
.rho..sub.c, .rho..sub.s, and .rho..sub.p of each of the
piezoelectric ceramics sheets 1-11, the shim plate 12, and the
covering members 1-2 as well as the width w of the piezoelectric
bimorph element 1-1 and is given by the following Equation (3).
.rho.S=2w(.rho..sub.pt.sub.p+.rho..sub.ct.sub.c+.rho..sub.st.sub.s)
(3)
[0045] When the covering member 1-2 as a new layer is added to each
of the opposite surfaces of the piezoelectric bimorph element 1-1,
the flexural modulus K and the weight .rho.S per unit length are
changed so that the resonance frequency is affected. Depending upon
the material selected as the covering members 1-2, the resonance
frequency of the acoustic vibration generating element may become
high. In most cases, however, the resonance frequency is lowered.
Specifically, if the covering members are formed by a flexible
material having an elastic modulus not greater than a predetermined
value, for example, a rubber having a small elastic modulus of
3.times.10.sup.6 Pa to 8.times.10.sup.6 Pa, the flexural modulus K
of a whole of the acoustic vibration generating element is
increased by addition of the new layer. However, an increasing rate
of the flexural modulus K is small as compared with an increasing
rate of the weight .rho.S per unit length. As a result, the
resonance frequency of the acoustic vibration generating element is
lowered.
[0046] FIG. 3 shows the change in resonance frequency when a
silicone rubber is used as the flexible material and a covering
thickness of the flexible material is changed. From FIG. 3, it is
understood that the flexible material has an effect of lowering the
resonance frequency and simultaneously lowering the mechanical
quality factor Q so that a frequency range is widened.
Consideration will be made of an acoustic output of the acoustic
vibration generating element as a bone conduction speaker. As the
acoustic vibration generating element radiates acoustic energy in
tight contact with a human skin, such a structure that the
piezoelectric element is covered with the flexible material is
suitable in view of acoustic impedance matching between the
acoustic vibration generating element and the human skin. In
particular, in case where the piezoelectric bimorph element is
covered with the flexible material, an effect of suppressing sound
leakage, i.e., radiation of unnecessary sound to the surroundings
is achieved.
[0047] In the foregoing, description has been directed to the case
where the acoustic vibration generating element has a rectangular
shape. It is noted here that the similar effect is obtained in case
where a piezoelectric bimorph or unimorph element of a circular
shape is used.
[0048] Hereinafter, this invention will be described in conjunction
with several preferred embodiments.
First Embodiment
[0049] [Piezoelectric Bimorph Element of a Rectangular Shape]
[0050] Preparation was made of a piezoelectric bimorph element
having a rectangular shape and comprising two piezoelectric
ceramics sheets (manufactured by NEC Tokin under the trade name of
NEPEC10.RTM.) having the length of 32 mm, the width of 8 mm, and
the thickness of 0.15 mm and a shim plate of brass having the
length and the width equal to those of the piezoelectric ceramics
sheets and the thickness of 50 .mu.m. The piezoelectric bimorph
element has a structure in which the shim plate is adhered between
the two piezoelectric ceramics sheets by the use of an epoxy
adhesive. Hereinafter, the above-mentioned structure will be called
a single-plate structure.
[0051] On the other hand, as illustrated in FIG. 4, another
piezoelectric bimorph element was produced in the following manner.
Preparation was made of two sets of laminated piezoelectric
ceramics members 5-1. Each of the laminated piezoelectric ceramics
members 5-1 comprises three piezoelectric ceramics sheets 5-11 and
two internal electrodes 5-12 interposed between adjacent ones of
the piezoelectric ceramics sheets 5-11. Each of the piezoelectric
ceramics sheets 5-11 is made of a material same as that of the
above-mentioned piezoelectric ceramics sheet and having the length
and the width equal to those of the above-mentioned piezoelectric
ceramics sheet and a thickness of 50 .mu.m. Between the two sets of
the laminated piezoelectric ceramics members 5-1, a metal shim
plate 5-14 is interposed through internal electrodes 5-13. Each of
the laminated piezoelectric ceramics members 5-1 has an outer
surface provided with an external electrode 5-15. A first one of
the internal electrodes 5-12 is connected to the internal electrode
5-13 via a side surface electrode 5-16. A second one of the
internal electrodes 5-12 is connected to the external electrode
5-15 via a side surface electrode 5-17. In the following, the
above-mentioned structure will be called a laminated structure. The
above-mentioned structure will hereinafter be called a laminated
structure.
[0052] In case of the piezoelectric bimorph element of the
single-plate structure, lead wires are connected to outer surfaces
of the shim plate and the piezoelectric ceramics sheets on opposite
outermost surfaces through the electrodes so that, when an electric
field is applied to one of the piezoelectric ceramics sheets in a
direction same as a polarization direction, a reverse electric
field is applied to the other piezoelectric ceramics sheet.
Similarly, in case of the piezoelectric bimorph element of the
laminated structure, lead wires are connected so that, when an
electric field is applied to one of the piezoelectric ceramics
sheets in the direction same as the polarization direction, a
reverse electric field is applied to another piezoelectric ceramics
sheet adjacent thereto.
[0053] Next, by the use of a brass die, a solution of silicone
rubber as the flexible material was poured over an entire surface
of the piezoelectric bimorph element. By a setting or curing
process, acoustic vibration generating elements of a single-plate
structure and a laminated structure were produced. Each of the
acoustic vibration generating elements was provided with rubber
covering having the thickness of 2 mm on two surfaces perpendicular
to the thickness direction and the thickness of 1 mm on remaining
surfaces.
[0054] When the acoustic vibration generating element of the
single-plate structure was supplied with an acoustic signal of
about 18 Vrms and one surface of the acoustic vibration generating
element was pressed against a user's head, a clear sound by bone
conduction was confirmed. The acoustic vibration generating element
of the laminated structure produced an output of the equivalent
level in response to an input of about 6 Vrms corresponding to 1/3
as compared with the single-plate structure.
[0055] Further, in order to evaluate a leaking sound, a sound
pressure of 100 Hz to 10 kHz was measured at a distance of 50 cm in
an anechoic chamber. As a result, it has been confirmed that, in
each of the single-plate structure and the laminated structure, a
sound pressure level was not greater than 50 dB. Thus, sound
leakage was very small.
[0056] Next, in order to quantitatively confirm acoustic effects of
the flexible material, an artificial internal ear (Artificial
Mastoid Type 4930 manufactured by B & K) was used to measure
and compare accelerations at a position corresponding to an
auditory nerve of a human body before and after covering with the
flexible material. It is noted that the magnitude of acceleration
in the internal ear is proportional to the strength of the acoustic
signal received by the auditory nerve.
[0057] FIG. 5 shows a result of comparison of acceleration (G) in
the artificial internal ear. As is obvious from FIG. 5, it has been
confirmed that, in case of the acoustic vibration generating
element covered with the flexible material, the acceleration is
improved in an output in a low-frequency region and the sharpness
of a resonant portion is considerably alleviated.
Second Embodiment
[0058] [Piezoelectric Bimorph Element of a Circular Shape]
[0059] Preparation was made of a piezoelectric bimorph element
having a circular shape and comprising two piezoelectric ceramics
sheets (manufactured by NEC Tokin under the trade name of
NEPEC10.RTM.) having the diameter of 30 mm and the thickness of
0.15 mm and a shim plate of brass having the diameter equal to that
of the piezoelectric ceramics sheets and the thickness of 50 .mu.m.
The piezoelectric bimorph element has a structure in which the shim
plate is adhered between the two piezoelectric ceramics sheets by
the use of an epoxy adhesive. Hereinafter, the above-mentioned
structure will be called a single-plate structure.
[0060] By the use of piezoelectric ceramics sheets made of a
material same as the above-mentioned piezoelectric ceramics sheets
and having the same diameter and the thickness of 50 .mu.m, a
circular piezoelectric bimorph element of a laminated structure was
prepared in the manner similar to that described in conjunction
with FIG. 4.
[0061] Wire connection was carried out in the manner similar to the
first embodiment.
[0062] Next, by the use of a brass die, a solution of silicone
rubber was poured over an entire surface of the piezoelectric
bimorph element. By a setting or curing process, acoustic vibration
generating elements of a single-plate structure and a laminated
structure were produced. Each of the acoustic vibration generating
elements was provided with rubber covering having the thickness of
2 mm on opposite surfaces perpendicular to the thickness direction
and the thickness of 1 mm on remaining surfaces.
[0063] The acoustic vibration generating element of the
single-plate structure and the acoustic vibration generating
element of the laminated structure were supplied with acoustic
signals of about 18 Vrms and about 6 Vrms, respectively. One
surface of each of the acoustic vibration generating elements was
pressed against a user's head. As a result, a clear sound by bone
conduction was confirmed.
[0064] Further, in order to evaluate a leaking sound, a sound
pressure of 100 Hz to 10 kHz was measured at a distance of 50 cm in
an anechoic chamber. As a result, it has been confirmed that, in
each of the single-plate structure and the laminated structure, a
sound pressure level was not greater than 50 dB. Thus, sound
leakage was very small.
[0065] Next, in order to quantitatively confirm acoustic effects of
the flexible material, an artificial internal ear (Artificial
Mastoid Type 4930 manufactured by B & K) was used to measure
and compare accelerations at a position corresponding to an
auditory nerve of a human body before and after covering with the
flexible material. It is noted that the magnitude of acceleration
in the internal ear is proportional to the strength of the acoustic
signal received by the auditory nerve.
[0066] FIG. 6 shows a result of comparison of acceleration (G) in
the artificial internal ear. As is obvious from FIG. 6, it has been
confirmed that, in case of the acoustic vibration generating
element covered with the flexible material, the acceleration is
improved also in a low-frequency region and the sharpness of a
resonant portion is considerably alleviated. The effects of
lowering the resonance frequency, decreasing the mechanical quality
factor Q, and preventing sound leakage by the flexible material can
be achieved not only by molding the silicone rubber as the flexible
material but also by adhering the flexible material to a surface of
the piezoelectric element.
Third Embodiment
[0067] [Covering member with V-shaped Grooves on Its Surface]
[0068] The acoustic vibration generating element experimentally
prepared in the first embodiment was subjected to mechanical
machining to form a plurality of V-shaped grooves on two principal
surfaces of the covering member of the flexible material (silicone
rubber in the embodiment). Each of the V-shaped grooves has a depth
of 0.6 mm and extends in a direction perpendicular to a lengthwise
direction. Thus, an acoustic vibration generating element according
to a third embodiment of this invention was produced.
[0069] FIG. 7 shows the acoustic vibration generating element
according to the third embodiment. The piezoelectric bimorph
element 1-1 is covered with the covering member 1-2'. The covering
member 1-2' is provided with a plurality of V-shaped grooves 6-1 on
its two principal surfaces.
[0070] The acoustic vibration generating element illustrated in
FIG. 7 was subjected to measurement using the artificial internal
ear in the manner similar to that described in conjunction with the
foregoing embodiments.
[0071] FIG. 8 shows the result of comparison of acceleration (G) in
the artificial internal ear. As is obvious from FIG. 8, the
acoustic vibration generating element having the V-shaped grooves
has an acceleration slightly greater than that of the acoustic
vibration generating element without the V-shaped grooves. The
sound leakage had an equivalent level. This is because the presence
of the V-shaped grooves facilitates bending and deformation so that
the flexural modulus K is apparently decreased, resulting in an
increase in generated force and in output level. Further, the
output level is increased in a low frequency region. This is an
effect similarly obtained by the decrease in flexural modulus K.
Specifically, as will be understood from Equation (1), the
resonance frequency F.sub.r is lowered due to the decrease in
flexural modulus K. The shape of the grooves formed on the surface
of the covering member is not limited to the V shape. The similar
effect is obtained by the grooves having semicircular section or
any other appropriate shape.
Fourth Embodiment
[0072] [Covering member with Air Chamber formed on One Side]
[0073] To one surface of the acoustic vibration generating element
experimentally prepared in the second embodiment, a circular ring
of soft rubber (having the outer diameter of 30 mm, the inner
diameter of 25 mm, and the thickness of 1 mm) equal in diameter to
the acoustic vibration generating element and a circular plate
(having the diameter of 30 mm and the thickness of 1 mm) of the
same material were successively adhered by the use of a
rubber-based adhesive. Thus, an air chamber having the diameter of
25 mm and the thickness of 1 mm was formed on one side of the
acoustic vibration generating element. Thus, an acoustic vibration
generating element according to a fourth embodiment of this
invention was produced.
[0074] FIG. 9 shows the acoustic vibration generating element
according to the fourth embodiment. The piezoelectric bimorph
element 1-1 is covered with the covering member 1-2' of the
flexible material. On one side of the acoustic vibration generating
element, the air chamber 8-1 is formed. An output surface, i.e.,
the other side of the acoustic vibration generating element without
the air chamber 8-1 was pressed against a part of a user's head and
an acoustic signal was supplied. By presence of the air chamber
8-1, sound leakage from an opening surface on the one side is
reduced.
[0075] In this embodiment, the air chamber 8-1 was formed by the
rubber ring and the circular plate. Not being limited thereto, the
air chamber may be formed in any other appropriate manner, for
example, may be formed integrally with the covering member. Even in
a structure such that the air chamber is connected to external air
in the process of molding, the similar effect is obtained. As will
readily be understood, the above-mentioned effect is obtained not
in the piezoelectric bimorph element of a circular shape but also
in the piezoelectric bimorph element of a rectangular shape similar
to that described in the first embodiment.
Fifth Embodiment
[0076] [Covering member and Ear Hook are Integrally Molded by
Flexible Material]
[0077] FIG. 10 shows an acoustic vibration generating element
comprising the piezoelectric bimorph element 1-1 experimentally
produced in the first embodiment and an ear hook 1-9 and the
covering member are integrally molded by a covering silicone
rubber.
[0078] FIG. 11 shows a state where the acoustic vibration
generating element in FIG. 10 is attached to a human ear 1-10. As
illustrated in FIG. 11, the acoustic vibration generating element
is attached to the human ear 1-10. When an electric signal is
supplied, a cartilage of an external ear and a cranial bone behind
the ear are simultaneously stimulated so that a sound by bone
conduction can be sensed more clearly.
[0079] Each of the first through the fifth embodiments has been
described in connection with the acoustic vibration generating
element in which a whole of the piezoelectric bimorph element is
covered with the covering member. However, the similar effect is
obtained in an acoustic vibration generating element in which the
piezoelectric bimorph element is covered with the covering member
on at least two surfaces perpendicular to the thickness direction
thereof. Of course, this applies to the piezoelectric unimorph
element.
[0080] As described above, this invention provides the acoustic
vibration generating element which is capable of lowering the
resonance frequency, decreasing the mechanical quality factor Q,
and preventing the sound leakage. The acoustic vibration generating
element has a robust and light-weight structure and has a wide
frequency range. Therefore, the acoustic vibration generating
element according to this invention is suitable for a bone
conduction device, in particular, to a bone conduction speaker.
[0081] While this invention has thus far been described in
connection with the preferred embodiments thereof, it will be
readily possible for those skilled in the art to put this invention
into practice in various other manners without departing from the
scope of this invention.
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